User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0
Steps to reproduce:
I have a videofile, encoded with x264 and MP3 audio. I put the same streams once in a MP4v1 (IS0 14496-12:2003) and once in a MP4v2 (ISO 14496-14) container. Here is the output of the "file" command:
VIDEOv1.mp4: ISO Media, MP4 Base Media v1 [IS0 14496-12:2003]
VIDEOv2.mp4: ISO Media, MP4 v2 [ISO 14496-14]
I'm currently using FF 78.7.0esr (64-bit)
When opening the file directly (using file://), the video was displayed properly. But the audio was only available on the MP4v1 container. In the MP4v2 container, the video kept silent (of course I verified, there was no mute setting). In the tab, the "sound icon" was also only visible with the MP4v1 version.
The audio should be played with both versions. Since I created both files from the same source and I only copied the streams, I am sure the streams themselves are correct.